Wednesday, 29 October 2008
Madison Ballroom (Hilton DeSoto)
The gale at 8 degree had occurred in 34 counties and cities from northern to southern in Hunan Province during the Period from 21:00,16th to 05:00,17th, on April 2007 (UTC, similarly hereinafter).Furthermore, there were total 6 weather stations which were observed two gale periods within one day., the maximum wind speed reached 27 m/s, in this event, the most active phase was during 01:00 to 04:00,17th, and the southern part of Hunan province was the gale dominated area. In this article, the gale process is analysed based on the synoptic data, the physical diagnosis products and the Doppler weather radar observations.The results show that, the low pressure trough at 500hPa, which was ahead of the shears in the middle and lower layers, companied with strong meridional circulation and followed by the strengthened cold temperature advection, led the ground cyclone wave move rapidly from north to south, and trigged a severe convection weather process in Hunan Province; The shears in the middle and lower layers stayed constantly over the southern, as the same time, a Jet-stream with more than 12m/s wind in 850hpa then located in the southern of Changsha city. Due to the downloading of the dynamical momentum in middle and lower layers, and the ground inverted trough frontogenesis, the beneficial weather conditions for this gale event were created. From the main three physical factors of CAPE, the Showalter's Index (SI), and the K index, we can see the SI and the K index in the southern of Hunan were obviously bigger than that of the northern. The CAPE value was 300.1J in Chenzhou weather station which located at southern Hunan, while the CAPE values were 0 in the other two stations of Zhangjiajie and Yueyang which located at the northern. Furthermore, through analyzing other three-dimensional physical quantity structure of the humidity, the temperature disturbance, the kinetic energy disturbance, the vorticity, the divergence and the vertical velocity, the every big value area was all located at below 700hPa.Thus, we only analyzed the middle and lower weather system including the surface cold front°¢the convergence in the middle and lower levels, those were the main trigger mechanism of the event, while the high-level divergence aspiration effect was relatively weak. On the radar echo figures, at the low level (0.5°,1.5°) (Fig.1a)there were many narrow band echoes with the width of 10-20km, but the length of several hundred kilometers. The echo cells dispersed and had not merge strengthened obviously. The echo scope of the middle level (1.5°-4.3°) (Fig.1b, Fig.1c ) increased along with its height, and the echo scope was probably the biggest in the 5-7km altitude, its area was approximately 2-3 times than the low level area's, then along with the height increasing the echo area reduced(Fig.1d), the strong echo band top was approximately 10km, the echo intensity which exceeded 50dBz located at below 5km, it was the low center of mass echo, because of the cold air excreted rapidly, and led the short-term thunderstorm gale, and it was not easy to form the severe convection weather like hail. High-level echo increased along with height, and inclined from northwest to southeast thanks to the northwest wind in high-level. While the cold air sank rapidly in high-level in front of the echo band, the thunderstorm gale occurred easily in the low level echo's front. On the Doppler velocity chart, low level speed chart which corresponded with the narrow band echo presented convergence line and unorganized negative speeds, but it did not achieve the meso-cyclone standard, and the high-level divergence was not obvious, thus created the strong convection weather, not getting to the supercell storm's intensity, mainly by thunderstorm gale weather. The great VIL value area which was bigger than 20kg/m2 moves to the southeast rapidly, it was typically gale echo characteristic. The above analysis indicated that: the ground cold front, the middle and low level convergence line are main trigger mechanism of this thunderstorm gale process, but the high-level divergence aspiration effect is relatively small, which is the primary reason of this process of which convection intensity is strong, but duration is not long.
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